1. Field of the Invention
The present invention relates to a charging method for a secondary battery and a device therefor and, more particularly, to a charging method during initial charge-discharge cycles after production of the secondary battery and a device therefor. Further, the present invention relates to a charging method for a secondary battery and a device therefor in cases where the secondary battery utilizes an oxidation-reduction reaction of lithium ions.
2. Related Background Art
In recent years, it has been determined that an increase in CO2 in the atmosphere causes the so-called greenhouse effect which, in turn, leads to global warming. Further, it should be noted that it is not easy to construct new thermal power plants. It is thus believed that so-called load leveling should be employed to store night-generated power in secondary batteries installed in ordinary homes in order to utilize power generators more effectively. Demands are increasing more and more for the development of high energy density secondary batteries for electric vehicles free of emission of air pollutants and for the high-performance secondary batteries as power supplies for portable equipment such as notebook type personal computers, word processors, video cameras, cellular phones, and so on.
Lithium (Li) secondary batteries making use of the electrochemical oxidation-reduction reaction of lithium have a higher operating voltage and lower weight than nickel-cadmium secondary batteries and lead second batteries which have been used heretofore. Thus, these Li batteries are drawing attention as high energy density type secondary batteries.
High-capacity Li secondary batteries using metallic lithium for the anode have an extremely short cycle life in charge and discharge cycles and are not yet put to practical use. It is generally believed that the principal cause of the short cycle life of charge and discharge is that the metallic lithium reacts with impurities of water, etc. and an organic solvent in an electrolyte solution to form an insulating film. This causes the lithium to grow into dendrites (dendrite crystals) with repetition of charge and discharge, and they establish an internal short circuit between the anode and the cathode, so as to cut short cycle life.
On the other hand, development is proceeding in so-called xe2x80x9clithium ion batteriesxe2x80x9d of the rocking chair type using as a cathode substance a lithium intercalation compound which deintercalates lithium ions from between layers in the oxidation-reduction reaction during charge and using as an anode substance a carbon material typified by graphite which can intercalate lithium ions between layers of six membered ring network planes comprised of carbon atoms. In this xe2x80x9cLi ion battery,xe2x80x9d however, the anode made of the carbon material of the graphite structure can theoretically intercalate at most only one sixth a lithium atom per carbon atom, and it is thus unsuccessful in realizing a high energy density secondary battery comparable to the Li primary batteries using the metallic lithium as an anode substance.
There are also proposals on methods of making an anode of a material selected from a lithium alloy, or a metal forming an alloy with lithium during charge, for example, aluminum, cadmium, indium, tin, antimony, lead, bismuth, or the like, alloys of these metals, or substances absorbing and desorbing lithium during charge and discharge. Examples of the substances absorbing and desorbing lithium during charge and discharge are silicon, metal oxides, and metal nitrides.
The Li secondary batteries using the metallic lithium, the carbon materials, the lithium alloys, the metals forming the alloy with lithium during charge, and the substances absorbing and desorbing lithium during charge and discharge are problematic in that the charge-discharge Coulomb efficiency does not reach 100% by initial charge and discharge after production of a battery. The reason is that there is an irreversible amount of lithium, because part of the lithium charged in the anode does not return to the cathode in the initial charge and discharge cycles after production of the battery.
This irreversible amount of lithium reduces the battery capacity of the Li secondary battery and is one of the causes of a shortened charge and discharge cycle life. There are thus desires for an activation method which restrains this irreversible amount.
The charging methods suggested heretofore include the following methods. A first suggestion is to specify the materials for the Li secondary battery and the charging method in order to suppress deposition of dendrites of the lithium metal. Specifically, Japanese Laid-open Patent Applications No. 5-114422 and No. 5-152002 disclose a method for making a battery using metallic lithium as an anode-active material, manganese dioxide as a cathode-active material, and a solution of lithium perchlorate dissolved in propylene carbonate as an electrolyte solution and for pulse-charging this battery; Japanese Laid-open Patent Application No. 7-263031 discloses a method for using metallic lithium as an anode-active material, the carbon-active material, and ethylene carbonate as a non-aqueous solvent of a non-aqueous electrolyte solution, and for pulse-charging the battery; Japanese Laid-open Patent Application No. 6-36803 discloses a method for carrying out pulse charging to repeat energization and stopping for the Li secondary battery using metallic lithium as an anode-active material. The above charging methods, however, have not increased cycle life in the practical area of lithium metal high energy density secondary batteries taking advantage of the intrinsic characteristics of the lithium metal, i.e., light weight and high capacity. None of the applications discloses anything about suppression of the irreversible amount of lithium in the initial charge and discharge.
Moreover, Japanese Laid-open Patent Application No. 9-117075, Japanese Laid-open Patent Application No. 6-290814 (U.S. Pat. No. 5,494,762), Japanese Laid-open Patent Application No. 8-45550 (U.S. Pat. No. 5,640,080), and Japanese Laid-open Patent Application No. 8-241735 (U.S. Pat. No. 5,640,080) disclose pulse charging methods as charging methods for shortening the charge time of lithium ion secondary batteries using carbon material for the anode. They all, however, fail to disclose a method for suppressing the above-described irreversible amount of lithium.
An object of the present invention is to provide an activation or charging method for a battery and a device therefor, solving the above problems, particularly, suppressing the non-dischargeable amount (the irreversible amount) in the initial stage of charge and discharge cycles after production of a battery, and improving the charge-discharge cycle characteristics in the secondary battery.
For accomplishing the above object, the present invention provides a charging method for a secondary battery at least comprised of an anode, a cathode, and an electrolyte and having an inflection point in a storage region before full charge (storage quantity 100%) in an anode potential curve or a cathode potential curve against storage quantity or in a curve of open-circuit voltage against storage quantity, wherein in carrying out a charging operation of the secondary battery by a charging current or a charging voltage of a predetermined waveform, the secondary battery is charged with variation of the charging current and/or the charging voltage, at least prior to arrival at the inflection point. Further, the charging method of the present invention is a method for charging the secondary battery by changing a charging pattern before to one after the storage quantity at the inflection point, i.e., by changing the waveform of the charging current or the waveform of the charging voltage before to one after the storage quantity at the inflection point.
The present invention also provides a charging device for a secondary battery for charging the secondary battery, said device comprising: a) a connection portion at least capable of electrically being connected to input and output terminals of a cathode and an anode of the secondary battery; b) charging means for charging the secondary battery via the connection portion; c) variation means for effecting variation of a charging current and/or a charging voltage of the charging means; and d) means for switching a charge pattern of the charging means before a predetermined charge quantity to another charge pattern after said charge quantity. The present invention further provides an activation device for a secondary battery for activating the secondary battery after assembly by charging and discharging the secondary battery, while providing the above charging device with e) means for discharging the secondary battery and f) means for switching electrical connection or disconnection between the secondary battery and the charging means or the discharging means.
Further, the present invention provides a charging device or a device for activating a battery of a cell pack structure wherein one or more secondary batteries connected in parallel or in series and held within a package, and a unit for communicating with the battery pack is provided.